1. Field of the Invention
The present invention relates to sound decoders, and more particularly, to a sound decoder for decoding, for example, a MUSE sound signal of NI-DPCM (Nearly-Instantaneous Differential Pulse Code Modulation) format that is multiplexed in a high definition television signal of MUSE (Multiple Sub-Nyquist Sampling Encoding) system format.
2. Description of the Background Art
In a high definition television signal of MUSE system (referred to as a MUSE signal hereinafter) proposed by Nippon Hoso Kyokai (NHK), a MUSE sound signal of NI-DPCM format (referred to as an NI-DPCM sound signal hereinafter) is multiplexed within the vertical blanking period of a MUSE video signal. Therefore, a MUSE sound decoder is required for extracting and decoding the NI-DPCM sound signal from the MUSE signal in receiving and reproducing such MUSE signals.
A typical MUSE signal television receiver generally contains a MUSE sound decoder for extracting and decoding a NI-DPCM sound signal that is multiplexed in a MUSE signal transmitted via a broadcasting satellite and received by an antenna and a baseband signal (BS) tuner, or an externally supplied MUSE signal reproduced from a recording medium by a reproduction equipment such as a MUSE disk player or a MUSE-VCR. Such a MUSE sound decoder is incorporated not only in MUSE signal television receivers, but also in various video equipments that process MUSE signals, and is also used as a unit in various environments wherein MUSE signals are processed. Such a MUSE sound decoder is disclosed in, for example, Japanese Patent Laying-Open No. 2-11076.
Prior to the description of a MUSE sound decoder, the principle of forming an NI-DPCM sound signal to be multiplexed in a transmission MUSE signal will be described schematically. The formation of such an NI-DPCM sound signal is described in detail in IEEE TRANSACTIONS ON BROADCASTING December 1987, Volume BC-33, No. 4, pp. 188-196 "Sound Transmission for HDTV Using Baseband Multiplexing into MUSE Video Signal" by T. Takegahara et al.
FIG. 1 is a schematic block diagram of a typical structure of a MUSE sound encoder for forming an NI-DPCM sound signal. Referring to FIG. 1, an analog sound signal supplied from a sound signal source (not shown) via an input terminal 1 is converted into a digital sound signal by an A-D converter 2 (A-mode sound data: sampling frequency 32 kHz, number of quantizing bits 15 bits/B-mode sound data: sampling frequency 48 kHz, number of quantizing bits 16 bits). This digital sound signal is provided to an NI-DPCM encoder 3 where the A-mode sound data of 15 bits and the B mode sound data of 16 bits are compressed to differential data of 8 bits (8 ranges) and 11 bits (6 ranges), respectively. The details of NI-DPCM encoding is described in the above mentioned document, and will not be described here.
The NI-DPCM sound signal provided from NI-DPCM encoder 3 is word-interleaved in the sample word direction in a word interleave circuit 4 to be supplied to one input of an adder 5. An error correction code generated from an error correction code generating circuit 6 is supplied to the other input of adder 5 to be multiplexed into the NI-DPCM sound signal. Following a BCH error correction process by a BCH error correction circuit 7, the signal is bit-interleaved by a bit interleave circuit 8. The above mentioned interleave and error correction processes are described in the aforementioned document, and will not be described here. The bit-interleaved NI-DPCM sound signal is supplied to one input of an adder 9. A frame synchronizing signal having a predetermined bit pattern of 16 bits and a control code of 22 bits from a signal generating circuit 10 is applied to the other input of adder 9 to be multiplexed into the NI-DPCM sound signal. As a result, a sound data bit stream of 1.35 Mbit/seconds is provided at a frame frequency of 1 kHz from adder 9.
FIG. 2 shows the bit arrangement of 1 frame (1350 bits) in A-mode, for example. FIG. 3 shows the bit allocation on a bit interleave matrix. The head frame synchronizing signal of each frame shown in FIGS. 2 and 3 has a fixed bit pattern of 16 bits of "0001001101011110".
As shown in FIG. 1, the NI-DPCM sound signal of 1.35 Mbit/sec provided from adder 9 is frame-interleaved in an inter-frame interleave circuit 11 to be provided to a binary to ternary conversion circuit 12, where the signal is converted into data of ternary format. The ternary data provided from binary to ternary conversion circuit 12 is converted into ternary data having sampling rate of 12.15 MHz in a time compression circuit 13 to be provided to a sampling frequency conversion circuit 14. The NI-DPCM signal has the sampling rate converted from 12.15 MHz to 16.2 MHz in sampling frequency conversion circuit 14 to be supplied to one input of an adder 15. The other input of adder 15 is supplied with a digital MUSE video signal having sampling rate of 16.2 MHz from a video signal source (not shown), whereby the above-described NI-DPCM sound signal is multiplexed within the vertical blanking period thereof. As a result, a MUSE signal of 16.2 MHz is provided from adder 15 to be output via a terminal 16. Thus, a MUSE signal provided from a MUSE encoder is converted into an analog signal to be transmitted by a broadcasting radio wave, or to be recorded on a recording medium such as an optical disk or a magnetic tape.
FIG. 4 is a block diagram showing a structure of a conventional MUSE sound decoder for decoding a sound signal from a MUSE signal that is formed in the above-described manner.
Referring to FIG. 4, an analog MUSE signal received via a broadcasting satellite or reproduced from a recording medium by a reproduction equipment is supplied to an input terminal 21. The analog MUSE signal is then converted into a digital signal by an A-D converter 22 to be provided to a sound signal gate 23. The NI-DPCM signal multiplexed in the vertical blanking period of the MUSE signal is separated by sound signal gate 23 to be provided to a sampling frequency conversion circuit 24.
The NI-DPCM signal of ternary format has the sampling rate converted from 16.2 MHz to 12.15 MHz by sampling frequency conversion circuit 24 to be provided to a ternary to binary conversion circuit 25. The ternary signal of 12.15 MHz is converted into a binary signal by ternary to binary conversion circuit 25 to be provided to a time expansion circuit 26.
The binary signal is converted into a baseband signal of 1.35 Mbit/sec by time expansion circuit 26 to be provided to an interframe de-interleave circuit 27. Since the provided signal is interframe-interleaved by the aforementioned inter-frame interleave circuit 11 (FIG. 1), this signal is de-interleaved by interframe de-interleave circuit 27 to have the proper frame order. Then, the signal is provided to a bit de-interleave circuit 28 and a frame synchronization detection circuit 34.
Since the NI-DPCM signal provided to bit de-interleave circuit 28 is bit-interleaved by the aforementioned bit interleave circuit 8 (FIG. 1), this signal is bit de-interleaved so as to be restored to a signal arranged in the sample word direction. The bit de-interleaved NI-DPCM sound signal, produced by circuit 28, provided to an error correction circuit 29, where error correction is carried out according to a BCH code. Then, the signal is provided to a word de-interleave circuit 30. Since this signal is interleaved in the sample word direction by the aforementioned word interleave circuit. 4 (FIG. 1), this word interleaved signal applied to circuit 30 is word de-interleaved so as to remove the interleave of the sample word direction thus restoring the word interleaved signal to a signal with the original one frame.
The output of word de-interleave circuit 30 is provided to an NI-DPCM decoder 31 to be restored to a digital sound signal. The principle of decoding by an NI-DPCM decoder is described in detail in the aforementioned document. The digital sound signal provided from NI-DPCM decoder 31 is converted into an analog signal by a D-A converter 32, and from there, provided to an output terminal 37, via a mute circuit 33.
The output of interframe de-interleave circuit 27 is also provided to a frame synchronization detection circuit 34 to detect a frame synchronizing pattern of 16 bits for each frame of the NI-DPCM sound signal. In other words, if the 16 head bits of the NI-DPCM sound signal of each frame is determined, by frame synchronization circuit 34, to exactly possess a bit pattern of "0001001101011110," a frame synchronization detection flag is generated and provided to a frame synchronization protection circuit 35.
In an abnormal case where frame synchronization detection circuit 34 determines that the frame synchronization pattern is temporarily missing, or w an erroneous detection is made over a short period, frame synchronization protection circuit 35 protects the frame synchronization by generating an internal frame signal of the proper frame frequency over a predetermined frame synchronization protection time period. If the above described abnormal state still remains even after the lapse of the aforementioned predetermined frame synchronization protection time period, frame synchronization protection circuit 35 provides a mute signal to a mute circuit 33 to mute the analog sound signal provided from D-A converter 32. The mute of the output sound signal is canceled by the proper detection of a frame synchronizing pattern by frame synchronization detection circuit 34. Various timing signals used in the decoder are generated even during the above described abnormal state from a timing signal generating circuit 36 according to an internal frame signal provided from frame synchronization protection circuit 35.
When a MUSE signal is transmitted as a broadcasting radio wave, the signal to noise (S-N) ratio is often degraded due to an influence of an external environment, such as the weather. Although error corrections by BCH codes and various interleaves are possible for the transmitted sound data, per se, as described in association with FIG. 1, there is no error correction provided for the frame synchronizing pattern of 16 bits located at the head of each frame data. Therefore, there may be partial portions missing from the frame synchronization pattern, having a bit length of 16 bits, due to degradation of S-N ratio. Because frame synchronization detection circuit 34 of the MUSE sound decoder generates a frame synchronization detection flag only when a complete frame synchronizing pattern of 16 bits is received, and does not generate a frame synchronization detection flag even if one bit is missing, there will be cases where the frame synchronization is frequently not detected in the case of degradation of S-N ratio due to aggravation of the radio wave by the weather. If the predetermined frame synchronization protection time period for the generation of the aforementioned mute signal is reduced, and set to a time period of 5 frames, for example, the mute will be applied frequently at a short cycle and even for sufficiently audible output sound signals which include some noise due to S-N ratio degradation, thereby resulting in disagreeable sound.
If the frame synchronization protection time period is lengthened, for example to 64 frames, for the purpose of suppressing frequent mute of such sound output, a problem is encountered that will be described hereinafter. A reproduced MUSE signal, provided from a reproduction equipment such as a MUSE disk player or a MUSE-VCR, has a constant S-N ratio, so that the abnormal state of not detecting frame synchronization owing to continuous S-N ratio degradation in a broadcasting radio wave is unlikely to occur. However, dropout easily occurs in the reproduced MUSE signal due to variation in the mechanical system of the reproduction equipment. Also, sound data as well as the synchronizing signal of a MUSE video signal may be missing completely over several frames at the time-of special reproduction or halt. In these cases, a significant amount of noise is generated. Although dropout of a relatively short time period can be compensated, owing to various error correction processes applied to the NI-DPCM sound signal, as described above, dropout over a long time period can not be compensated. It is effective to apply mute to sound output for such a missing signal. However, a long frame synchronization protection time period (for example, a time period of 64 frames) results in the generation of a substantial amount of noise since muting of the sound output is not immediately effective when dropout occurs in a sound signal.